Conventionally, as shown in FIG. 10, there is known a coaxial connector 500 comprising a center conductor 510 and a tubular housing 520 disposed around the center conductor 510 (see, e.g. JP-A-2007-323865 (Patent Document 1)). The housing 520 has an inner periphery formed with a convex portion 521m which is adapted to engage with convex portions 623 formed on an outer periphery of a tubular portion 621 of a coaxial adapter 600 which is adapted to be inserted into the coaxial connector 500. The tubular portion 621 of the coaxial adapter 600 is formed with a plurality of slits 622. Upon inserting the coaxial adapter 600 into the coaxial connector 500, the convex portions 623 of the tubular portion 621 abut against the convex portion 521m of the housing 520 so that the diameter of the tubular portion 621 is reduced. Then, as the coaxial adapter 600 is further pushed forward, the convex portions 623 of the tubular portion 621 ride over the convex portion 521m of the housing 520 so that the tubular portion 621 is elastically restored to increase its diameter. As a consequence, the convex portions 623 of the tubular portion 621 and the convex portion 521m of the housing 520 engage each other so that the coaxial adapter 600 is prevented from coming off the coaxial connector 500.